In the production of steel, the refractory brick linings of conventional process vessels generally wear unevenly due to hot spots, slag attack, or other operating parameters. In the ongoing attempt to minimize production costs, a major concern of steel makers is the wearing of the refractory brick lining of the vessel and the resultant consumption of refractory composition to replace the lining. In order to extend the useful life of such linings, it has become acceptable practice to repair excessively worn areas between heats by means of gunning granular refractory compositions onto the brick surfaces. For example, a dry mixture including granular refractory grain is pneumatically delivered to a nozzle of a spray gun where it is mixed with water, resulting in a bonded composition that is projected against the irregularly worn surface, thereby filling in or patching the unevenly worn refractory lining with the refractory composition. Under the usual circumstances, gunning is initiated within minutes after the molten steel is tapped from the hot vessel. When properly constituted and emplaced the gunning mixture is a relatively stiff pliable mass that adheres to the hot vessel lining and which quickly converts to a relatively hard non-pliable mass. As the temperature of the furnace rises during the next heat, components of the mixture more permanently bond the refractory grains to themselves and to the unevenly worn lining.
The granular refractory material used in gunning mixtures is typically composed of magnesia having a periclase crystalline phase. Such materials include dead burned natural magnesite and a synthetic periclase product derived from seawater or brine. Because of their comparatively lower cost in the market, dead burned dolomite and/or chrome ore grog, along with dead burned magnesite in different combinations, have also been conventionally employed as refractory material in gunning mixtures. However, due to the lack of any bonding when wetted with water, refractories must additionally include a binder to provide at least temporary strength to the composition throughout its entire use temperature range (110 degrees Celsius to 1760 degrees Celsius). Optionally, a clay mineral may also be included in the mixture as a gelling or plasticizing agent.
Both organic and inorganic binders have been employed for use in gunning mixtures. Representative organic binders have included starch, dextrin, various organic sulfonic acids and salts, and tars, pitches and resins, but all of these binders suffer a variety of drawbacks, which render them detrimental for use in gunning mixtures. For example, refractory mixtures containing organic sulfonic acids as a low temperature binder have been found to ignite and burn when applied between heats to the refractory brick lining of a hot (1200 degrees Celsius) metallurgical vessel, thereby compromising their ultimate utility. For the most part, organic additives contribute excessive porosity to the refractory composition thereby diminishing the repaired linings durability and strength.
Inorganic binders also pose problems when employed in gunning mixtures. Solid inorganic binders capable of being dissolved or dispersed in water have been in common use in gunning mixtures, and typical of these are chromic acid, sodium silicate, phosphate glasses, Epsom salts, and magnesium chloride. As with the organic additives, most of these binding agents suffer one or more disadvantages such as slowness of reaction or air setting, lack of strength at intermediate temperatures, poor slag resistance to name a few.
Silicates, phosphates and sulfates are generally the binders of choice for bonding magnesia-based refractory compositions, due primarily to their generally acceptable mechanical strength, slag resistance, and durability of the resultant products. Steel producers are constantly looking for compositions that will increase durability relative to current technology. This would decrease downtime and reduce the frequency of maintenance the metallurgical vessel requires for repair.
Therefore, what has been found to be new and novel is a refractory composition that improves upon the performance characteristics, such as hot strength, slag resistance, and overall durability, compared to current compositions being used to produce or repair refractory linings.
It is an objective of the present invention to provide a magnesia-sulfamic acid-calcia product having improved hot strength. Another objective of the present invention is to provide a magnesia-sulfamic acid-calcia refractory product exhibiting outstanding refractoriness i.e. this composition will not form deleterious low melting compounds that are formed when traditionally bonded compositions are used. A further objective of the present invention is to provide a magnesia-sulfamic acid-calcia refractory product outstanding in overall durability.
Additional objectives and features will become apparent from the following detailed description of the embodiments of the present invention.